Capacitive micromachined ultrasonic transducers (CMUTs) for transmitting/receiving ultrasonic waves have been proposed. A CMUT is prepared by means of a micro electromechanical systems (MEMS) process devised by applying semiconductor processes. FIG. 9A is a schematic cross-sectional view of a CMUT array and connections therein and FIG. 9B is a schematic illustration of a CMUT array on a substrate. FIG. 9A is a cross-sectional view taken along 9A-9A in FIG. 9B. The CMUT array in FIGS. 9A and 9B includes vibration films 101, first electrodes (upper electrodes) 102, supporting portions 103, gaps 104, second electrodes (lower electrodes) 105, a substrate 106 and insulating films 107. In FIG. 9B, each cell 200 is formed by a vibration film 101 and a supporting portion 103. A first electrode 102 is formed on the vibration film 101 and the vibration film 101 is supported by the supporting portion 103 that is formed so as to be disposed on the substrate 106. A second electrode 105 is arranged on the substrate 106 at a position opposite to the first electrode 102 formed on the vibration film 101, with a gap 104 (normally having a thickness of tens to hundreds of several nanometers) interposed between them. The surface of the substrate 106 on which CMUTs are formed (the upper surface in FIG. 9A) is referred to as CMUT-forming surface and the opposite surface (the lower surface in FIG. 9A) is referred to as non-CMUT-forming surface hereinafter. A set of a vibration film 101 (normally having a thickness of about tens of several micrometers), a first electrode and a second electrode, which is arranged opposite to the first electrode with a gap 104 interposed between them, is referred to as a cell 200. A plurality of cells in which the first electrodes 102 and the second electrodes 105 are respectively electrically connected is collectively referred to as an element 201 that operates as a unit for transmitting/receiving ultrasonic waves. The first electrodes and the second electrodes in the element are drawn out to the outside of the CMUT array by respective wiring lines 108 and 109 provided outside the element. Since insulating films 107 are arranged on the substrate 106 and the substrate 106 and the wiring lines 108 and 109 are mutually insulated, the wiring lines of different elements are also mutually and electrically insulated.
A DC potential applying unit 301 can apply a DC potential to the first electrodes (upper electrodes) 102 that are mutually connected by way of the wiring lines 108. As a result of making the first electrodes to show a predetermined potential by means of the DC potential applying unit 301, a predetermined DC potential difference VB is produced between each of the first electrodes and the oppositely disposed second electrode 102. Because of the potential difference VB between the electrodes, when the electrodes receive ultrasonic waves and the distance between them is changed accordingly, a dielectric current (charge) is generated on the electrodes. Then, electrostatic attraction is generated at the electrodes to bend the vibration film 101 toward the substrate 106 due to the predetermined potential difference VB between the electrodes. Consequently, the distance between the electrodes is reduced to raise the transmission/reception efficiency of the ultrasonic transmitting/receiving operation of the CMUT. A drive detecting unit 302 has at least one of an AC voltage applying unit for applying an AC voltage to the electrodes and a current detection unit for detecting the electric current generated at the electrodes. Therefore, the drive detecting unit 302 can apply an AC potential to the first electrodes (upper electrodes) 102 by way of the wiring lines 108 or detecting the generated electric current. Thus, by generating electrostatic attraction between the two electrodes by means of the applied AC voltage, the drive detecting unit 302 can operate for generating vibrations and transmitting elastic waves. The drive detecting unit 302 can also operate for detecting the magnitude of arriving elastic waves, detecting the change in the electric charge (electric current) due to a change in the capacity of the vibration film that is vibrating due the elastic waves.
As described above, wiring lines 108 and 109 need to be arranged for the purpose of exchanging electric signals from the electrodes with the outside, and hence the elements 201 of a CMUT array are arranged with certain intervals separating them.